There are electrophotographic image forming apparatuses in which the amount of developer contained in a development device decreases as it is consumed in image development, and fresh developer is supplied from a developer container to the development device when the amount of developer therein falls to or below a predetermined amount. Thus, the amount of developer therein is kept in a given range. Additionally, the developer container and the development device may be housed in a common unit casing, forming a single development unit removably installed in the image forming apparatus. When the amount of developer contained in the development unit falls to or below the predetermined amount, the development unit is replaced as a whole.
Such configurations require a detector to detect the amount of developer inside the development device or development unit. Therefore, various types of detectors have been proposed to detect the amount of developer. For example, light transmission-type detectors including optical elements are used to detect the amount of developer.
Light transmission-type developer amount detectors determine the amount of developer in the developer container based on the amount of light transmission therein.
In this method, light emitted from a light-emitting element can be guided to a light-receiving element using first and second light guides provided inside the developer container across a clearance. The first and second light guide are constructed of, for example, prisms or mirrors. When the amount of developer in the developer container is sufficient, a light path formed between the first and second light guides is blocked by the developer, and the light-receiving element does not receive the light. However, when the amount of developer in the developer container is reduced to or below a reference amount, the developer does not block the light path, and the light can reach the light-receiving element. It can be determined whether the amount of developer has decreased below the reference amount by measuring the output from the light-receiving element (as disclosed in JP-2007-219269-A, JP-4358038-B, and JP-4398421-B).
The development unit, an image bearer such as a photoreceptor, and the like may be housed in a common unit casing, forming a modular unit (i.e., a process unit), which is typically longer in the axial direction of the photoreceptor. In such process units, the amount of developer tends to be uneven in an end portion in its longitudinal direction. Accordingly, it is preferred to detect the amount of developer in a center portion in the longitudinal direction, in which the amount of developer is relatively uniform.
Depending on the layout of the development unit, the photoreceptor, and the like, however, it is difficult to dispose the light-emitting element and the light-receiving element in the center portion in the longitudinal direction. For example, in an arrangement in which the development unit is above the photoreceptor, it is difficult to provide a separate positioning member around the development unit for fixing the light-emitting element and the light-receiving element in position. Therefore, the light-emitting element and the light-receiving element are disposed on a side wall of the image forming apparatus adjacent to an end of the development unit in the longitudinal direction.
Although the amount of developer in the center portion of the development unit can be detected using a light guide extending from the end portion to the center portion of the development unit to guide the light from the light-emitting element, it is possible that the light is attenuated while passing through the long light guide. Accordingly, light-emitting elements of higher output power are required, thus increasing the cost.